Geosynthetic-Reinforced Embankments Over Soft Foundations
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Geosynthetics - 7 th ICG - Delmas, Gourc & Girard (eds) © 2002 Swets & Zeitlinger, Lisse ISBN 90 5809 523 1 Geosynthetic-reinforced embankments over soft foundations R. K. ROWE and A.L. LI, GeoEngineering Centre at Queen’s-RMC Department of Civil Engineering, Queen’s University, Kingston, Ontario, Canada ABSTRACT: The behaviour of reinforced embankments over conventional soft cohesive soil, rate sensitive soil and peat deposits is reviewed and recent design and analyses methods are summarized. The findings from both field observations and finite element analyses are presented. Both undrained and partially drained behaviour of reinforced embankments are considered. The use of reinforcement in combination with prefabricated vertical drains is addressed. The effects of both the viscous and inviscous characteristics of reinforcement and foundation soils on embankment behaviour are discussed. It is concluded that the partial consolidation provided by PVDs and the tension mobilized in reinforcement can substantially increase embankment stability. However, creep of geosynthetics can decrease the embankment failure height. The mobilization of reinforcement during and after embankment construction can vary significantly depending on the soil and reinforcement characteristics. Particular care must be taken for designs where the foundation soil is rate sensitive. mechanisms can be identified for reinforced embankments: INTRODUCTION lateral sliding of embankments over the base reinforcement The behaviour and design of geosynthetic reinforced layer, foundation extrusion (bearing capacity failure), rotational embankments over soft soil has attracted considerable attention slope failure involving breakage or pull-out of reinforcement, in both practice and in the literature. The behaviour of basal and excessive displacement. reinforced embankments over typical soft soils is now well If the interface shear strength between the reinforcement understood and a number of papers have addressed these issues (e.g. geosynthetic) and fill is inadequate, active earth pressure (e.g. Humphrey and Holtz 1987; Jewell 1988; Rowe 1997; within the embankment may cause the embankment to slide Leroueil and Rowe 2001; Rowe and Li 2001) and books such as laterally on top of the reinforcement although, in practice, this is Jewell (1996) have summarized some common design rarely a critical case. Alternatively, with shallow deposits of low methodologies. However it has also been found that these strength soil, the foundation material can be laterally extruded design methods may be overly conservative for conventional from beneath the reinforced embankment. If the reinforcement soils and may be unconservative for less conventional soils. The is placed directly on the foundation material then this mechanism objectives of this paper are six fold. First, to summarize typical may involve horizontal movement of the foundation soil relative methods of analysis and design for reinforced embankments. to the reinforcement and the overlying embankment. The key Second, to discuss the performance of reinforced embankments parameters controlling these two mechanisms are the shear as observed in field cases. Third, to discuss the issues strength of the foundation soil and the reinforcement-soil surrounding the selection of “compatible strain” as a criterion for interface strength in direct shear. the design of reinforced embankments. Fourth, to examine the The factor of safety against a rotational slip failure may be effect of partial drainage and the use of prefabricated vertical increased by the inclusion of geosynthetic reinforcement. The drains (PVDs) on the design and performance of reinforced tensile force required to maintain stability must be developed in embankments on soft clay and to present a new design the reinforcement by means of shear stresses between the methodology. Fifth, to summarize design considerations and reinforcement and the soil above and below it. Once the methods for embankments on fibrous peat. Finally, to provide interface shear strength is reached, the reinforcement will pull- new insights regarding the performance and design of reinforced out of the soil and rotational failure will occur. Alternatively, if embankments on rate sensitive soils and to examine the potential the tensile strength of the reinforcement is reached, the breakage effect of creep in the reinforcement itself. of the reinforcement will result in a rotational failure. These two possibilities are fairly obvious. There is, however, a third and somewhat less obvious potential failure mechanism. The METHODS OF ANALYSIS embankment may fail at a reinforcing force lower than that expected based on stability considerations due to the stress- Jewell (1988) provided a rational understanding of the role of the strain-time characteristics of the reinforcement. If the reinforcement in embankments on soft soil. Key to these reinforcement has a low mobilized tensile stiffness, J, then large considerations was the recognition that the lateral earth pressure deformations of the foundation may occur prior to reinforcement within an embankment over a soft cohesive foundation imposes failure. Under these circumstances, it may not be possible to shear stresses on the foundation soil which reduces the bearing construct the embankment to the desired height even though capacity of the foundation and hence embankment stability. The "collapse" has not occurred. Furthermore, for some soils, basal reinforcement can serve to resist some or all of the earth significant movement along the potential failure surface may pressure within the embankment and to resist the lateral result in strain softening of the soil. Additional load will then be deformations of the foundation thereby increasing bearing transferred to the reinforcement leading to even larger strains capacity and stability. A number of idealized failure until eventually the reinforcement will break accompanied by 5 Sommery Search Author Index Sommery Search Author Index failure of the embankment. In order to prevent this failure synthesized the bearing capacity factors of Davis and Booker mechanism, consideration must be given to: (a) the (1973) and Matar and Salencon (1977) for rough footings (Fig. reinforcement-soil interface shear strength under conditions 1) and proposed a simple method of estimating the stability of a where the reinforcement is pulled out from between the soil highly reinforced embankment. This approach, which will be above and below it; (b) the tensile strength of the reinforcement; briefly outlined here, considers the effect of increasing undrained and (c) the stress-strain characteristics of the reinforcement shear strength with depth as well as the effect of the relative relative to those of the foundation soil. thickness of the underlying cohesive soil deposit. A reinforced Embankments on highly compressible foundations may fail embankment can never be reinforced beyond the point of being due to excessive displacements. For a particular geometry and rigid, hence these solutions place an upper limit on the soil profile, there is a threshold reinforcement tensile stiffness improvement in stability that can be achieved using high below which the reinforcement has no effect upon settlement. strength/tensile stiffness reinforcement. For reinforcement with tensile stiffness (modulus) in excess of Since an embankment will generally be trapezoidal in shape this threshold value, the reinforcement will reduce lateral and the plasticity solutions are for a rigid footing of width b, an spreading and local yield. This effect will be greatest for approximation must be made to obtain the equivalent width of shallow deposits or for deposits where the soil strength and the embankment. From plasticity considerations, the pressure at modulus increase with depth. However, the reinforcement the edge of a rigid footing is (2+ π)suo, where suo is the undrained cannot eliminate settlement. For each geometry there is also an shear strength directly beneath the footing. It is assumed here upper threshold tensile stiffness above which any further that the effective width of the footing b will extend between the increase in reinforcement stiffness does not alter the settlement. points on either side of the embankment when the applied * Thus, under some circumstances, excessive deformations may pressure γh is equal to (2+π)suo. Thus occur even if a high tensile stiffness reinforcement is used. This * possibility must be recognized at the design stage and, if h = (2+π)suo/γ (1) necessary, consideration should be given to the use of a lightweight fill material (e.g. see Rowe and Soderman 1985b, and hence (from Fig. 2) 1986). Many embankments are constructed on deposits with a b = B + 2n(H-h*) (2) relatively stiff crust or root mat overlying a weaker and more compressible main deposit. This crust/root mat is a natural where suo = the undrained shear strength at the top of the deposit, reinforcement and will contribute significantly to embankment γ = weight of the fill, B = the crest width, H = the embankment stability while reducing settlements. However, in doing so, height, and n = the cotangent of the slope angle (see Fig. 2). tensions may develop within the crust/root mat. If the limited The bearing capacity qu of the equivalent rigid footing of tensile strength of the crust is exceeded, tensile failure will occur width b is given by followed by the embankment sinking into the soft underlying soil. In these cases, the major role of the reinforcement